The present invention relates to a silicon device used in inertial force sensor or the like, and particularly to a silicon device comprising an insulating substrate and a beam-like structure made of silicon formed on the insulating substrate.
Recently it has been made possible to etch silicon as deep as 100 xcexcm by reactive ion etching technology using an inductively coupled plasma (ICP) as the activation energy source (hereinafter referred to as ICP-RIE process). This technique is viewed as a promising new technique for making silicon structures of high aspect ratios with a sufficiently high etching rate, in the field of device development by micromachining. In the past, wet processing using an alkaline solution was predominant as the process for deep etching of silicon substrates. But it is difficult to make a desired structure by wet processing, because the direction of etching depends on the crystalline orientation of silicon in the wet process. In contrast, the ICP-RIE process is not subject to the anisotropy of etching because it is a dry process. Thus the ICP-RIE process has such an advantage over the wet processing that far higher degree of freedom in the design of the configuration of structure can be achieved than in the case of wet processing.
However, such problems as described below have been encountered when the ICP-RIE process is applied to the manufacture of an inertial force sensor such as acceleration sensor or angular velocity sensor having such a structure as a beam-like structure made of silicon in the form of cantilever, simple beam or the like is supported on an insulating substrate made of glass or the like.
FIG. 12 shows the structure of an inertial force sensor 100 as an example of the fundamental structure of a silicon device of the prior art. FIG. 13A through FIG. 13F schematically show the manufacturing process of the inertial force sensor 100. Similar manufacturing process has been proposed, for example, by Z. Xiao et al. in Proc. of Transducers ""99, pp.1518-1521, and S. Kobayashi et al. in Proc. of Transducers ""99, pp.910-913.
FIG. 12 is a schematic plan view and FIG. 13F is a sectional view taken along lines XIII-XIIIxe2x80x2 of FIG. 12. The inertial force sensor 100 includes an insulating substrate 101 that has a recess 102 formed in the surface thereof, a beam-like structure 104 made of silicon bonded onto the surface of the insulating substrate 101 so as to interpose the recess therebetween and a frame 108 that surrounds the beam-like structure 104 made of silicon with a space kept therefrom and is bonded onto the insulating substrate 101. The beam-like structure 104 further includes two electrodes 105, 105xe2x80x2. The electrodes 105, 105xe2x80x2 include a supporting section 106 and a plurality of cantilevers 107, a supporting section 106xe2x80x2 and a plurality of cantilevers 107xe2x80x2, respectively. The cantilevers 107 and 107xe2x80x2 are arranged to oppose each other via a minute clearance.
A silicon substrate 103 is provided in the step of FIG. 13A, and the glass substrate 101 is provided in the step of FIG. 13B. A mask film is formed on the surface of the glass substrate 101 by the photolithography process, and a recess 102 is then formed by etching the surface of the glass substrate 101 to a depth in a range from several micrometers to several tens of micrometers by means of a diluted solution of hydrofluoric acid in the step of FIG. 13C. In the step of FIG. 13D, the silicon substrate 103 is bonded onto the surface of the glass substrate 101 by anodic bonding method. In the step of FIG. 13E, a mask film 109 having a pattern that corresponds to the planar configuration of the beam-like structure 104 shown in FIG. 12 is formed on the surface of the silicon substrate 103 by the photolithography. In the step of FIG. 13F, the silicon substrate 103 is etched through by the ICP-RIE process, thereby to form of the beam-like silicon structure 104 and the frame 108. Then the resist remaining on the surface of the silicon substrate is removed.
The step of FIG. 13F involves such a problem as described below. The mask film 109 shown in FIG. 13E generally has both wide apertures and narrow apertures. Consequently, when a dry etching process such as the ICP-RIE process is applied to the silicon substrate 103 that has the mask film 109, the silicon substrate is etched at a higher rate in a portion exposed through the wider aperture than in a portion exposed through the narrower aperture due to the micro loading effect. As a result, the portion of the silicon substrate 103 exposed through wider aperture is etched through earlier than the portion exposed through narrower aperture. At this time, etching gas enters into the clearance between the recess 102 of the glass substrate 101 and the back surface of the silicon substrate 103 through the hole which has been etched out in the silicon substrate 103 earlier. The etching gas that has entered erodes the back surface of the silicon substrate 103 till the portion exposed through the narrowest aperture is completely etched out. Thus the side wall of the supporting section 106 and the bottom surface or the side wall of the cantilever 107 are eroded. As a result, dimensions of the beam-like structure 104 deviate significantly from the design values, making it impossible to obtain the target characteristics of the device and resulting in lower reliability.
The present applicant found that the problem described above is caused by positive charging of the recess of the insulating substrate by the etching gas that has positive charge. Accordingly, the present applicant proposed a method for suppressing the erosion of the beam-like silicon structure by providing the recessed portion with an electrically conductive film that has electrical continuity with the supporting section (M. Chabloz, J. Jiao, Y. Yoshida, T. Matsuura, K. Tsutsumi, A Method to Evade Microloading Effect in Deep Reactive Ion Etching for Anodically Bonded Glass-Silicon Structures, Proc. MEMS2000, pp.283-287, Miyazaki, Japan, 2000). However, there is still a demand to further suppress the erosion of the beam-like silicon structure in order to improve the reliability.
There is also such a problem that an attempt to make the aperture sizes equal for the purpose of eliminating the microloading effect leads to a significant decrease in the degree of freedom in the design of the device structure.
Even when the aperture sizes are set to be the same in design, it is difficult to completely prevent the erosion of the supporting section and the cantilever in the actual process. This is because it is a common practice to apply over-etching to some extent in order to etch through reliably. In the case of an acceleration sensor, for example, a cantilever of a movable electrode and a cantilever of a fixed electrode are arranged to oppose each other via a minute clearance, with the minute clearance being formed in such a pattern as the width increases and decreases repetitively. The sensor has higher sensitivity as the ratio the clearance of the larger width to the clearance of the smaller width becomes higher. When the ratio becomes too high, however, the etching rate varies significantly from point to point over the surface due to the microloading effect, thus resulting in a lower etching rate in the narrow clearance region. This makes it necessary to apply over etching to the narrow clearance region, that causes more damages on the back surface of the silicon substrate during the etching process.
An object of the present invention is therefore to provide a silicon device that has higher reliability and offers a sufficient degree of freedom in the design of the device structure, by suppressing the erosion of the beam-like silicon structure due to the micro loading effect.
A silicon device of the present invention includes an insulating substrate having a recess formed on the surface thereof, a beam-like structure made of silicon formed on the front surface of the insulating substrate to surround the recess, said beam-like structure including at least one functional section having a supporting section bonded onto the insulating substrate and at least one cantilever formed integrally with the supporting section while extending across the recess; a frame made of silicon that surrounds the beam-like structure with a space kept therefrom and is formed onto the insulating substrate; and a conductive film having electrical continuity with the frame and formed on the surface of the insulating substrate at least in a portion right below the cantilever.
The silicon substrate is etched in such a mechanism of dry etching as activated ions having positive charge are accelerated by a negative bias formed right above the silicon substrate thereby to collide onto the silicon substrate with a sufficient energy. In the case of the ICP-RIE process, sulfur fluoride ion (SFx+) is usually used as the activated etching gas. The ion changes into silicon fluoride (SiFx) through reaction with silicon, and is discharged to the outside. The negative bias is formed immediately above the silicon substrate by applying a high frequency field to a substrate holder that also serves as a cathode whereon the silicon substrate is placed. Therefore, erosion of the back surface of the silicon substrate is considered to occur as the SFx+ that has entered the clearance between the back surface of the silicon substrate and the recess of the insulating substrate is repulsed by the surface of the insulating substrate and collides with the back surface of the silicon substrate. Repulsion of the SFx+ on the surface of the insulating substrate may be caused also by electrical repulsion force as well as kinematic scattering.
FIG. 9A and FIG. 9B are schematic sectional views showing a silicon substrate 53 bonded onto the surface of an insulating substrate 51, which has a recess, so as to surround the recess 52, in a state of the silicon substrate 53 being dry-etched. The silicon substrate 53 has a mask film 59 formed on the surface thereof for the purpose of forming a functional section. The silicon substrate 53 is formed into a beam-like silicon structure 56 and a frame 58 through dry etching, while the beam-like silicon structure 56 is further formed into a movable electrode 57 and a fixed electrode 57xe2x80x2. The movable electrode 57 comprises cantilevers 572, 572 and a supporting section 571 that supports the cantilevers, while the fixed electrode 57xe2x80x2 comprises cantilevers 572xe2x80x2, 572xe2x80x2 and a supporting section 571xe2x80x2 that supports the cantilevers
During the dry etching process, the surface of the recess 52 of the insulating substrate 51 is charged with positive charge 62 by the etching gas, for example, SFx+61 which impinges thereon a number of times. The surface of the recess 52 charged with the positive charge repulses the following SFx+61. The repulsed SFx+61 changes the direction of the movement thereof before reaching the recess 52 and instead impinges on the back surface of the silicon substrate 53, thereby eroding the cantilever 572, 572xe2x80x2. Also it may be that the SFx+61 which would otherwise be bound to hit the insulating substrate 51 at right angles is distracted from the trajectory by the recess 52 that is positively charged, and impinges on and erodes the side walls of the supporting sections 571, 571xe2x80x2. Therefore, in order to restrict the erosion of the back surface of the silicon substrate 53 or the supporting sections 571, 571xe2x80x2, it is effective to prevent the surface of the recess 52 of the insulating substrate 51 from being positively charged.
FIG. 10A and FIG. 10B are schematic sectional views showing the structure of the silicon device proposed by M. Chabloz et al. mentioned above, that is similar to the structure shown in FIG. 9A except for the electrically conductive film 54 that has electrical continuity with the supporting section 571 via an electrical continuity section 55a, and is formed on the surface of the recess. When the etching gas 61 collides with the electrically conductive film 54, the electric charge dissipates through the supporting section 571 so as to become inactive. During dry etching, the silicon substrate 53 is held at a negative potential, the same potential as the substrate holder (not shown). Therefore, collision of the etching gas onto the electrically conductive film 54 neutralizes the charge thereby accelerating the deactivation of the etching gas. This enables it to significantly restrict erosion of the back surface of the silicon substrate 53. But the cantilever 572xe2x80x2 was made smaller than the cantilever 572 as shown in FIG. 10B. This is supposedly caused because positive charge 62 that has migrated from the electrically conductive film 54 through the supporting section 571 charges the cantilever 572 positively, thus repelling the etching gas which passes near the cantilever 572 thereby forcing it to impinge on the cantilever 572xe2x80x2 so as to erode it. Although the degree of damage resulting from charging of the silicon cantilever is lower than that caused by charging of the insulating substrate, it may cause damage to the base portion of the cantilever and lead to lower reliability depending on the mask pattern.
According to the present invention, the electrically conductive film 54 that has electrical continuity via the electrical continuity section 55b with the frame 58 is formed on the surface of the recess.
Since the frame 58 is separated from the beam-like silicon structure 54 and is bonded to the insulating substrate 51, the positive charge does not migrate from the electrical conductive film 54 into the cantilever. Moreover, there is no minute structure such as cantilever that requires high precision machining near the frame 58. Therefore, even if the frame 58 is charged, it does not cause damage to the minute structures.
Furthermore, linkage of the frame 58 with the frames of adjacent devices is maintained until processing of the wafer has been completed and the wafer separated into individual devices by dicing, and the frame has the largest volume among the silicon structures formed on the wafer. As a result, amount of charge per unit volume (volume charge density) can be minimized compared to a case of connecting to other silicon structure on the wafer. Thus it is made possible to minimize the repulsion force against the etching gas that passes nearby, thereby to further restrict the erosion of the beam-like silicon structure by the etching gas.
The beam-like structure of the silicon device according to the present invention may also include two or more functional sections that are electrically insulated from each other and have substantially the same volumes.
The silicon device of the present invention is an acceleration sensor that includes an insulating substrate having a recess formed on the surface thereof, a beam-like structure made of silicon formed on the front surface of the insulating substrate to surround the recess, and a frame made of silicon that surrounds the beam-like structure with a space kept therefrom and bonded onto the insulating substrate, wherein the beam-like structure includes a movable electrode and a fixed electrode with the movable electrode and the fixed electrode each having a supporting section bonded onto the insulating substrate and a comb-shaped electrode that consists of a plurality of cantilevers formed integrally with the support section and extending across the recess, while the cantilevers of the movable electrode and the cantilevers of the fixed electrode are disposed to oppose each other via a minute clearance, and an electrically conductive film which is electrically connected with the frame is formed on the surface of the insulating substrate at least in a portion right under the cantilever.
In the acceleration sensor described above, the movable electrode and the fixed electrode may have substantially the same volumes.
The silicon device of the present invention is an angular velocity sensor that includes an insulating substrate having a recess formed on the surface thereof, a beam-like structure made of silicon formed on the front surface of the insulating substrate to surround the recess, and a frame made of silicon that surrounds the beam-like structure with a space kept therefrom and bonded onto the insulating substrate, wherein the beam-like structure includes a movable electrode and a fixed electrode with the movable electrode and the fixed electrode each having a comb-shaped electrode that consists of a plurality of cantilevers extending across the recess, while the movable electrode is supported on the frame so as to be capable of vibrating in the horizontal direction on the surface of the insulating substrate and the fixed electrode is bonded onto the insulating substrate, with the cantilevers of the movable electrode and the cantilevers of the fixed electrode being disposed to oppose each other via minute clearance, and an electrically conductive film which is electrically connected with the frame is formed on the surface of the insulating substrate at least in a portion right under the cantilever.
The silicon device of the present invention includes an insulating substrate having a recess formed on the surface thereof, a beam-like structure made of silicon formed on the front surface of the insulating substrate to surround the recess, and a frame made of silicon that surrounds the beam-like structure with a space kept therefrom and is bonded onto the insulating substrate, wherein the beam-like structure has at least one functional section that includes a supporting section bonded onto the insulating substrate and at least one cantilever that is formed integrally with the supporting section and extending across the recess, while the beam-like silicon structure includes two or more functional structures that are electrically insulated from each other and have different volumes, and an electrically conductive film that has electrical continuity with the supporting section of the functional section of the largest volume is formed on the surface of the insulating substrate at least in a portion right below the cantilever. Even when the functional section of the largest volume is electrically charged, volume charge density of this functional section can be made the lowest among the functional sections. Thus it is made possible to minimize the repulsion force against the etching gas that passes nearby, thereby to further restrict the erosion of the beam-like silicon structure by the etching gas.
The silicon device of the present invention is an acceleration sensor that includes an insulating substrate having a recess formed on the surface thereof, a beam-like structure made of silicon formed on the front surface of the insulating substrate to surround the recess, and a frame made of silicon that surrounds the beam-like structure with a space kept therefrom and bonded onto the insulating substrate, wherein the beam-like structure includes a movable electrode and a fixed electrode with the movable electrode and the fixed electrode each having a supporting section bonded onto the insulating substrate and a comb-shaped electrode that consists of a plurality of cantilevers formed integrally with the supporting section and overhanging into the clearance region, while the cantilevers of the movable electrode and the cantilevers of the fixed electrode are disposed to oppose each other via minute clearance, the movable electrode and the fixed electrode being formed to have different volumes, and an electrically conductive film which is electrically connected with the supporting section of either the movable electrode or the fixed electrode that has the larger volume is formed on the surface of the insulating substrate at least in a portion right under the cantilever.
The silicon device of the present invention can be manufactured by separating a silicon wafer, which is used as the silicon substrate whereon a number of silicon devices have been formed, into individual silicon devices by dicing. For example, in a method for manufacturing a silicon device including an insulating substrate having a recess formed on the surface thereof, a beam-like structure made of silicon formed on the front surface of the insulating substrate to surround the recess, and a frame made of silicon that surrounds the beam-like structure with a space kept therefrom and is bonded onto the insulating substrate, with the silicon device having at least one functional section in which the beam-like structure includes a supporting section bonded onto the insulating substrate and at least one cantilever that is formed integrally with the supporting section and extending across the recess, the silicon device can be manufactured in the process including a step of forming the electrically conductive film on the surface of the recess at least in a portion right under the cantilevers and extending the electrically conductive film over the surface around the recess thereby to form the electrical continuity section with the frame, a step of forming a first mask film on the surface of the silicon substrate in a pattern corresponding to the configuration of the supporting section, a step of etching the surface of the silicon substrate whereon the first mask film is formed so as to form the supporting section, a step of bonding the silicon substrate that has the supporting section and the insulating substrate that has the electrically conductive film so that the both surfaces oppose each other, a step of forming a second mask film on the back surface of the bonded silicon substrate in a pattern corresponding to the configuration of the cantilevers, and a step of dry etching the back surface of the silicon substrate whereon the second mask film has been formed so as to penetrate through the silicon substrate thereby to form the cantilevers extending across the recess in a desired pattern.